1. Field of the Invention
The present invention relates to a fuel cell system utilizing a solid-oxide fuel cell, in particular to a fuel cell system capable of variably controlling the fuel utilization ratio.
2. Description of the Related Art
A control method of a conventional polymer electrolyte fuel cell system is described. FIG. 3 shows a conventional polymer electrolyte fuel cell system. As illustrated in FIG. 3, when the fuel cell system is operated, a fuel processing device 14 reforms a material such as natural gas using steam to generate a gas containing hydrogen as a main component. The gas is humidified by a hydrogen-side humidifier 11 and supplied to a fuel cell 1. Also, an air-supply device 3 supplies air as an oxidation gas to an oxidation-side humidifier 13, and the oxidation gas is humidified thereby and supplied to the fuel cell 1. The fuel cell 1 is connected to a power conditioner 6 that converts generated DC power to AC power. The power conditioner 6 is connected to an electric power system 7 and an electrical load 8.
The heat which is generated by the fuel cell 1 accompanying the power generation is recovered by cooling water flowing in a cooling pipe 19. The cooling water is circulated by a cooling water circulation pump 16 and the heat recovered by the cooling water is transferred to water which is circulated by a pump 17 in an exhaust heat recovery pipe 20 through a heat exchanger 15. Thus, hot water is reserved in a hot water storage tank 12 (refer to, for example, Japanese Unexamined Patent Publication (Kokai) No. 2002-42841).
The polymer electrolyte fuel cell is characterized in that the exhaust heat recovery efficiency is higher than the power generation efficiency. Therefore, in ordinary operation, the temperature of water is swiftly increased and the hot water storage tank is easily filled with hot water. In such a case, in view of the efficiency of the system that the exhaust heat recovery efficiency is higher, the power generation operation is suspended.
Also, the polymer electrolyte fuel cell is characterized in that city gas as a main fuel is reformed to generate hydrogen and carbon monoxide. Although the generated carbon monoxide is removed, there is a problem that considerable time is required to remove the carbon monoxide. Therefore, the fuel is hardly reformed swiftly corresponding to the power generation amount and/or temperature changes. Consequently, the fuel utilization ratio is controlled extremely slowly.
To solve the above-mentioned disadvantage, an exhaust heat recovery system for a polymer electrolyte fuel cell system has been developed. However, no exhaust heat recovery system has been proposed for a solid-oxide fuel cell system. Since an electrolyte is different from a solid electrolyte, the control relevant to the exhaust heat recovery for the polymer electrolyte fuel cell system cannot be applied to the solid-oxide fuel cell system as is.
The solid-oxide fuel cell is characterized in that the power generation efficiency is higher than the exhaust heat recovery efficiency. In view of electric power supply to the electrical load, the solid-oxide fuel cell is superior to the polymer electrolyte fuel cell. However, in view of stable supply of hot water to a load to be supplied therewith, the solid-oxide fuel cell has a disadvantage that the absolute suppliable amount of hot water is small.